1. Field of the Invention
The present invention relates to an apparatus for controlling an internal combustion engine (hereinafter referred to an engine) when the engine is required to decelerate a rotational speed.
2. Description of Related Art
JP-A-63-71539 discloses an idle speed control system (hereinafter referred to an ISC) having an ISC valve for varying an amount of air bypassing a throttle valve. In this apparatus, a target rotational speed is set relatively low to save fuel consumption when an actual rotational speed is slowly lowered. On the contrary, the target rotational speed is set relatively high to prevent an engine stall when the actual rotational speed is rapidly lowered. However, since the system has a delay, the target rotational speed must be set sufficiently high from a final target rotational speed to prevent a stall and a vibration of the engine. Such a high target rotational speed causes a delay on the ISC and increases fuel consumption.
The present invention addresses these drawbacks.
It is therefore an object of this invention to provide an apparatus for preventing a stall of the engine and improving fuel consumption.
It is a further object of this invention to provide an apparatus for controlling the rotational speed of the engine to a target rotational speed quickly.
According to a first aspect of the present invention, an air amount bypassing a throttle valve is decreased according to a difference between an actual rotational speed and a target rotational speed when the engine is operated under a predetermined decelerating state. Therefore, the rotational speed of the engine is quickly lowered.
According to a second aspect of the present invention, a feedback control is started after a completion of the decrease control. Therefore, the rotational speed can be stably maintained at the target rotational speed after a quick lowering by the decrease control.